1. Technical Field of the Invention
This invention relates generally to internal combustion engines, and more specifically to selectable firing order modes for such.
2. Background Art
The examiner is warned that the invention itself is not directly described until Page 8. The first six pages are a rather lengthy, but necessary, explanation of the prior art.
FIG. 1 illustrates a four-cylinder engine 10 which, for purposes of illustrating the present invention, has been drastically simplified. Well-known features such as gears, bearings, intake tracts, exhaust manifolds, fluids, oil pumps, transmissions, gearboxes, counter balancer shafts, gaskets, water jackets and channels, and so forth have been omitted. Those of ordinary skill in the art of internal combustion engines would not be aided by their inclusion, as they are largely irrelevant to the invention.
The four-stroke cycle is: intake, compression, power, exhaust. At the start of the intake stroke, the exhaust valve is closed and the intake valve is opened. The piston moves from TDC to BDC, filling the cylinder with fuel/air mixture. The intake valve is then closed, and the piston returns to TDC during the compression stroke. The spark plug is fired, igniting the fuel/air mixture, which expands and presses the piston back to BDC. The exhaust valve is opened, and during the exhaust stroke the piston returns to TDC, forcing the waste gasses out of the cylinder. For simplicity, such issues as valve overlap and timing advance/retard are ignored in this disclosure, as they are well-understood in the art, and their inclusion here would only render the invention harder to learn from this disclosure. The crankshaft makes two complete revolutions, rotating 720°, during each four-stroke cycle. Some engines have been built such that each spark plug fires not only at the start of its piston's power stroke, but also 360° later at the start of its piston's intake stroke. This intervening spark has generally been benign, as there is no fuel/air mixture present and essentially no compression in the cylinder.
The engine includes a lower crankcase 12 mated with an upper crankcase 14. A cylinder block 16 is coupled to the top of the upper crankcase, and includes cylinder bores 18. The lower crankcase, upper crankcase, and cylinder block are shown in cross-section or cutaway view, to facilitate viewing of the internal engine components. The head includes a set of intake valves (not visible) and exhaust valves (not visible). The intake valves are operated by a set of intake valve actuators 20. The exhaust valves are operated by a set of exhaust valve actuators 22 as shown. In most engines, the valve actuators consist of one or more camshafts; however, the present invention is far more easily practiced with individually controllable actuators such as pneumatic or hydraulic actuators, rather than actuators which operate as a set. The head further includes a set of spark plugs 24 and a set of fuel injectors 26. Many engines use carburetors instead of fuel injectors.
The engine includes a crankshaft 28 which, typically, is coupled between the lower crankcase and the upper crankcase. For each cylinder, there is a piston 30-1 through 30-4 which is coupled to the crankshaft by a connecting rod 32. As the pistons reciprocate up and down in their cylinders, their connecting rods drive the crankshaft to reciprocate around its axis.
FIG. 5 illustrates the crankshaft 28 in greater detail. The crankshaft is built around a center shaft 34. Four “big pins” 36-1 through 36-4 are offset from the axis of the center shaft by a distance which is equal to one half the stroke of the engine. (The displacement of the engine is equal to the number of cylinders*the cylinder radius squared*pi*the stroke.) Each big pin is coupled to the center shaft by two webs 38. The portion of the center shaft between the two webs is absent, or it would interfere with the connecting rod as the crankshaft turned. The remaining portions of the center shaft between adjacent big pins' webs form the “center pins” 40 of the crankshaft. The center pins and the end portions 42 of the crankshaft ride in bearings or bushings in the crankcases.
FIGS. 1 and 5 together illustrate that the respective angular positions of the big pins largely determine the firing order of the engine. The first piston 30-1 is at its top dead center (TDC) position, as is the fourth piston 30-4. The second piston 30-2 is at its bottom dead center (BDC) position, as is the third piston 30-3. The crankshaft is rotating such that its top is rotating toward the reader, out of the page, or clockwise as viewed from the end of the engine which is shown on the left.
This crankshaft 28 is known as a “flat” or “180°” crankshaft, because all four big pins lie in the same plane.
Because a four-stroke engine crankshaft makes two complete revolutions, rotating through 720°, during the four-stroke cycle of any given piston, there are a plurality of firing orders possible with any particular crankshaft. The term “firing order” refers not only to the numerical sequence in which the pistons/cylinders have their power strokes, but also the relative timing between the power strokes. Typically, the firing order is reported with cylinder one first in the order.
FIG. 9 illustrates one known firing order for a flat 180° four-cylinder crankshaft. The X axis indicates the crankshaft rotational position from 0° (with piston 30-1 at TDC) through two complete crankshaft revolutions, ending at 720°. Cylinder 1 (with piston 30-1) has its intake stroke from 0° through 180°, where it is at BDC. It then has its compression stroke from 180° through 360°, where it is again at TDC. It then has its power stroke from 360° through 540°, and its exhaust stroke from 540° through 720°.
Cylinder 2 (with piston 30-2) has its intake stroke from 180° through 360°, its compression stroke from 360° through 540°, its power stroke from 540° through 720°, and its exhaust stroke from 0° through 180°. Cylinder 3 (with piston 30-3) has its intake stroke from 540° through 720°, its compression stroke from 0° through 180°, its power stroke from 180° through 360°, and its exhaust stroke from 360° through 540°. Cylinder 4 (with piston 30-4) has its intake stroke from 360° through 540°, its compression stroke from 540° through 720°, its power stroke from 0° through 180°, and its exhaust stroke from 180° through 360°.
Thus, the four cylinders' power strokes come at exactly even 180° intervals, with the firing order 1-2-4-3. (Note that the firing order is always reported beginning with cylinder 1, rather than with the arbitrarily-selected 0° crankshaft position.) This configuration is sometimes referred to as a “screamer” engine. The equally-spaced power pulses will, in many engines, produce the least vibration. Big pins 36-1 and 36-2 are 180° out of phase; therefore, pistons 30-1 and 30-2 are always at exactly opposite relative positions, moving at exactly equal and opposite velocities, and undergoing exactly equal and opposite accelerations. The same is true of pistons 30-3 and 30-4. This gives a flat 180° crankshaft engine perfect “primary balance” (in the plane in which the pistons move).
FIG. 10 illustrates a different firing order which can be achieved with exactly the same crankshaft. The timings of Cylinders 1 and 2 are unchanged. The valve and ignition timings of Cylinders 3 and 4 are offset 360° relative to where they were in FIG. 9, such that Cylinders 2 and 3 operate together, and Cylinders 1 and 4 operate together. This configuration is sometimes referred to as a “big bang” engine. The one complete crankshaft revolution from 0° to 360° includes no power strokes. This is especially beneficial in some motorcycle applications, as it gives the rear tire time to regrip the asphalt before the next power pulses tend to make it break traction. There are two simultaneous power pulses (at 360°), then two more simultaneous power pulses 180° later (at 540°), then 540° before the next power pulse begins.
FIG. 6 illustrates a different four-cylinder crankshaft 50, in which the big pins 52 are differently positioned than in the flat 180° crankshaft described above. When the first cylinder's big pin 52-1 is at TDC, the second cylinder's big pin 52-2 is 90° after TDC, the third cylinder's big pin 52-3 is 90° before TDC, and the fourth cylinder's big pin 52-4 is at BDC. This crankshaft may be termed a “90°” crankshaft, because there is one big pin at every 90° position.
FIGS. 3 and 4 illustrate an engine 60 using the crankshaft 50. When the first piston 30-1 is at TDC, the second piston 30-2 is 90° after TDC, the third piston 30-3 is 90° before TDC, and the fourth piston 30-4 is at BDC. As shown in FIG. 4, when the first piston has advanced to 90° after TDC, the second piston is at BDC, the third piston is at TDC, and the fourth piston is 90° before TDC. The first and fourth pistons provide primary balance of each other, and the second and third pistons provide primary balance of each other (in position, velocity, and acceleration).
FIG. 11 illustrates one firing order achievable with the engine of FIGS. 3 and 4. Cylinder 1 is the reference, as above. Cylinder 2 has its intake stroke from 270° to 450°, its compression stroke from 450° to 630°, its power stroke from 630° to 90°, and its exhaust stroke from 90° to 270°. Cylinder 3 has its intake stroke from 450° to 630°, its compression stroke from 630° to 90°, its power stroke from 90° to 270°, and its exhaust stroke from 270° to 450°. Cylinder 4 has its intake stroke from 540° to 720°, its compression stroke from 0° to 180°, its power stroke from 180° to 360°, and its exhaust stroke from 360° to 540°. There are 270°between the Cylinder 1 and Cylinder 2 power pulses, 180°between the Cylinder 2 and Cylinder 3 power pulses, and 90° between the Cylinder 3 and Cylinder 4 power pulses, then 180°without a power pulse. The asymmetry of this 270/180/90/180 pulse train is why it is referred to as “odd pulse”.
FIG. 12 illustrates another firing order achievable with the engine of FIGS. 3 and 4. Cylinder 1 has its power pulse at 360°, Cylinder 3 90° later at 450°, Cylinder 4 another 90° later at 540°, and Cylinder 2 another 90° later at 630°, followed by 450° of rotation before the next power pulse begins. This 90/90/90/540 sequence is why it is referred to as “even pulse”—because the four power pulses are evenly spaced.
FIG. 7 illustrates a crankshaft 70 for use in a five-cylinder engine (not shown). With the first big pin 72-1 at TDC, the second big pin 72-2 is at 144° past TDC, the third big pin 72-3 is at 72° before TDC, the fourth big pin 72-4 is at 72° after TDC, and the fifth big pin 72-5 is at 144° before TDC. There is one big pin every 360°/5=72°, and they are arranged in the familiar five-pointed-star order.
FIG. 13 illustrates a “screamer” firing order for a five-cylinder engine (not shown) utilizing the crankshaft of FIG. 7. Cylinder 1 has its power stroke at 360°, Cylinder 4 has its power stroke 144° later at 504°, Cylinder 2 has its power stroke 144° after that at 648°, Cylinder 5 has its power stroke 144° later at 72°, and Cylinder 3 has its power stroke 144° after that at 216°. The power pulses occur regularly, one every 144°. The screamer firing sequence is 1-4-2-5-3 with this crankshaft.
FIG. 14 illustrates a “big bang” firing order using the same crankshaft as in FIG. 13. Cylinder 5 has its power stroke at 72°, Cylinder 4 has its power stroke 72° later at 144°, Cylinder 3 has its power stroke 72° later at 216°, Cylinder 2 has its power stroke 72° later at 288°, and Cylinder 1 has its power stroke 72° later at 360°. The firing of Cylinders 2 and 4 has been shifted 360° from that of FIG. 13, with the other Cylinders' firing left unchanged. The big bang firing sequence is 5-4-3-2-1.
FIG. 8 illustrates a crankshaft 80 for a six-cylinder engine (not shown). The crankshaft has the conventional 120° configuration, as is typically found in six-cylinder automobile engines. That is, when viewed end-on, there are two big pins every 120°, rather than one every 60°. With big pins 82-1 and 82-6 at TDC, big pins 82-2 and 82-5 are at 120° after TDC, and big pins 82-3 and 82-4 are at 120° before TDC.
FIG. 15 illustrates the common “screamer” firing order for an in-line six-cylinder engine. Cylinder 1 has its power stroke at 360°, Cylinder 5 at 480°, Cylinder 3 at 600°, Cylinder 6 at 0°, Cylinder 2 at 120°, and Cylinder 4 at 240°. The firing order is 1-5-3-6-2-4. Any two consecutive power strokes occur on opposite halves of the crankshaft, and one power stroke occurs every 120° of rotation. The in-line six-cylinder engine is extremely well balanced, both in the primary and secondary (perpendicular to the primary) forces.
FIG. 16 illustrates a “big bang” firing order achievable using the same crankshaft as in FIG. 15. The firing of Cylinders 2, 3, and 6 is shifted 360° relative to that of FIG. 15, such that a pair of power strokes occur at 240°, a pair 120° later at 360°, and a final pair 120° later at 480°, followed by 480° without a power stroke beginning.
Previously, in order to change a given engine with a particular crankshaft from one firing order to another, it was necessary to make a host of difficult, expensive, and time-consuming modifications to the engine. For example, it has typically been necessary to tear down the engine, perhaps first removing it from the vehicle, sufficiently far to remove and replace one or more of the camshafts. It has typically also been necessary to replace the distributor cap, or reprogram the engine control unit (ECU). Such changes in the firing order take advantage of the fact that each piston is at TDC twice during each 720° four-stroke cycle; by offsetting a particular cylinder's valve, spark, and fuel injector controls by 360°, a different firing order is achieved without having to split the crankcases and replace the crankshaft. In general, there are N2/2 possible firing orders for a given N-cylinder crankshaft, but only a limited number of these will be desirable; the others will suffer from balance or vibration problems.
What is desirable is an improved engine and an improved method of changing that engine's firing order, which is less difficult, less expensive, and less time-consuming than previous engines have permitted. Ideally, the firing order change should be able to be performed on the fly, such as by the vehicle's operator.